Two-Tap Equalizer for the Eye Skewing Distortion

Eye diagram skew is a well-known impairment observed at high baud rates in directly modulated lasers. Eye skew manifests as a time-shift between the lower and upper parts of the eye diagram in higher-order pulse amplitude modulation (PAM) signals. In this letter, we show that alike the inter-symbol interference can be explained by frequency-selective attenuation of the signal, the eye skewing is a result of a time shift between the fundamental frequency and the 2nd-order harmonic distortion generated in a nonlinear system. This reverse-engineering analysis leads to an ultra-simple two-tap eye (de)skewing equalizer, which only requires asynchronous operations easily implemented in analog circuitry: squaring, addition, and delay. The relationship between the two of the proposed equalizer parameters and the resulting skew is established theoretically, while the equalizer performance is validated in a 50 Gbit/s PAM-4 VCSEL transmission system.